Aspects of the present disclosure relate generally to wireless communication systems, and more particularly, to closed-loop feedback for continuous relative evaluation for uplink transmit diversity.
Wireless communication networks are widely deployed to provide various communication services such as telephony, video, data, messaging, broadcasts, and so on. Such networks, which are usually multiple access networks, support communications for multiple users by sharing the available network resources. One example of such a network is the UMTS Terrestrial Radio Access Network (UTRAN). The UTRAN is the radio access network (RAN) defined as a part of the Universal Mobile Telecommunications System (UMTS), a third generation (3G) mobile phone technology supported by the 3rd Generation Partnership Project (3GPP). The UMTS, which is the successor to Global System for Mobile Communications (GSM) technologies, currently supports various air interface standards, such as Wideband-Code Division Multiple Access (W-CDMA), Time Division-Code Division Multiple Access (TD-CDMA), and Time Division-Synchronous Code Division Multiple Access (TD-SCDMA). The UMTS also supports enhanced 3G data communications protocols, such as High Speed Packet Access (HSDPA), which provides higher data transfer speeds and capacity to associated UMTS networks.
For a mobile user in the HSPA (High Speed Packet Access) cellular system, the user experience is often limited by transmit power of User Equipment (UE). For instance, a cell edge UE has to transmit at a low data rate, or possibly not establish a call, due to transmit power limitations. The technique of closed loop transmit diversity (CLTD) is useful to improve these situations. Assume multiple transmit antennas are utilized in the UE. The UE transmitter can apply a weighting vector to the transmit antennas such that the signals from these antennas are coherently combined at the base node (e.g. “Node B”) receive antennas.
A reduction in the required UE transmit power due to beamforming gain from CLTD improves the link budget and the user experience. Furthermore, when the signals across different antennas experience independent fading, coherent signal combining results in a more stable composite channel with a smaller probability of deep fading. As such, beamforming can provide diversity gain. The motivation of considering the closed loop beamforming scheme is that, via the Node B processing and feedback, the UE transmitter can apply a beamforming phase to achieve the aforementioned gains (possibly at the expense of more complexity and more downlink feedback power). Since the UE forms the beam only toward the serving cell, the signals from the two UE transmit antennas are typically received at all other cells without constructive addition. Thus, from the network level point of view, the amount of interference caused by this UE at other Node B receivers is reduced. This interference reduction leads to network throughput improvement. However, there are scenarios where transmit diversity can result in detrimental performance.